The present invention relates to an electromagnetic flow meter, and, more particularly, to an electromagnetic flow meter of a type having a ceramic measuring pipe capable of satisfactorily measuring the flow rate of a hot fluid, a corrosive fluid or an abrasive fluid or the like.
The electromagnetic flow meter is based on the principle that the flow rate can be measured by detecting the electric potential, which is generated when a conductive fluid to be measured flows perpendicular to the magnetic field, and which is in proportion to the product of the intensity of the applied magnetic field and the flow rate of the fluid, the detection being performed by a pair of electrodes. The measuring pipe of the flow meter, which comes into contact with the fluid to be measured, has a rubber or an ethylene fluoride resin lining on the inner surface thereof in order to have corrosion resistance and to achieve an electrical insulation. In particular, electromagnetic flow meters of a type having a ceramic measuring pipe have been produced recently in order to improve the heat resistance and the wear resistance.
FIG. 1 illustrates the structure of a conventional ceramic electromagnetic flow meter. Referring to FIG. 1, reference numeral 1 represents a ceramic measuring pipe through which a fluid to be measured flows. The measuring pipe 1 has a pair of electrodes 4 facing each other in the radial direction thereof in such a manner that their leading portions project into the inside portion of the measuring pipe 1. The measuring pipe 1 has thick flange portions at the two end portions thereof so as to be secured to a case member 7 by an organic or inorganic adhesive at the outer circumferential surface portions of the flange portions. Electromagnetic coils 2a, 2b and cores 3 are disposed adjacent to the outer surface of the central portion of the measuring pipe 1, the electromagnetic coils 2a, 2b and the cores 3 acting to generate a magnetic field which penetrates the measuring pipe 1. When a fluid flowing in the measuring pipe 1 traverses the aforesaid magnetic field, an electric potential is generated in the fluid. The electric potential thus generated is detected by the pair of the electrodes 4. A wire ejection portion 9 is formed in the central portion of the case member 7 so that wires for exciting the electromagnetic coils 2a and 2b and wires, through which detection signals supplied from the electrodes 4, are connected to outer terminal portions.
The electromagnetic flow meter is clamped and fastened by bolts 12 and nuts 13 while interposing earth rings 5a and 5b and gaskets 10a and 10b disposed on the two sides of the earth rings 5a and 5b in a state where the electromagnetic flow meter is held between a pair of piping flanges 11a and 11b. The earth rings 5a and 5b are in contact with the fluid and are connected to an earth line of the electromagnetic flow meter so that an initial electric potential of the fluid and the earth potential of the electromagnetic flow meter are made to be the same. The gaskets 10a and 10b, in terms of fluid, seals the portions between the process piping flanges 11a, 11b and earth rings 5a, 5b and the portions between the earth rings 5a, 5b and the ceramic measuring pipe 1 of the electromagnetic flow meter.
However, the conventional electromagnetic flow meter encounters the following problems.
When the electromagnetic flow meter is mounted to the process piping, each of the bolts is fastened at a tightening torque of thousands N.cm so as to apply compressive force to each gasket in the axial direction (in the direction of the thickness) in order to obtain fluidtight performance. Since the aforesaid fastening or clamping force is received by the surface of each flange of the ceramic measuring pipe, the surface of the flange is applied with a large plane pressure of tens Mpa to hundreds Mpa.
The plane pressure thus applied generates compressive stress (FIG. 2) in the cylindrical portion of the ceramic measuring pipe in the axial direction and bending moments acting on the flanges. The bending moments result an axial tensile stress and a circumferential tensile stress.
In general, ceramics have the tensile strength about one-tenth of the compressive strength. Therefore, the aforesaid tensile stresses are the largest cause for the breakage of the ceramic measuring pipe. In actual practice, the ceramic measuring pipe is usually broken at positions shown in FIG. 3.
Therefore, an upper limit of the tightening torque for the bolt at the time of fastening the electromagnetic flow meter structured as shown in FIG. 1 to the process piping is usually specified. If the bolt is fastened with a tightening torque larger than the specified torque, the ceramic measuring pipe can be broken. Even if the bolts are fastened with a tightening torque smaller than the specified torque, a so-called "unevenly fastened state", in which only either of the bolts is tightly clamped, will generate excessive tensile stress, causing the ceramic measuring pipe to be broken.
Further, the gasket 10b held between the flange of the ceramic measuring pipe and the earth ring deforms outwardly due to the clamping force applied thereto, that is, a so-called "cold flow" takes place. As a result, another problem arises in that the pressure applied to the gasket is undesirably reduced and therefore the degree of the fluidtight sealing deteriorates.
Another problem arises in that excessively large thermal stress is generated in the flange portion of the ceramic measuring pipe if the temperature of the fluid, which flows in the ceramic measuring pipe, has been rapidly changed because the flange portion has a large thickness. It also leads to a fact that the ceramic measuring pipe is broken.
Accordingly, an object of the present invention is to provide an electromagnetic flow meter, which can be fastened to process piping while preventing generation of excessively large tensile stress in a ceramic measuring pipe thereof, and in which deterioration of fluidtight sealing performance due to cold flow of gaskets interposed between flanges of the ceramic measuring pipe and earth rings can be prevented.
Another object of the present invention is to provide an electromagnetic flow meter, which can be fastened to process piping while preventing generation of excessively large tensile stress in a ceramic measuring pipe thereof, in which deterioration of fluidtight sealing performance due to cold flow of gaskets interposed between flanges of the ceramic measuring pipe and earth rings can be prevented, and in which generation of excessively large thermal stress can be prevented even if temperature of the fluid has been changed rapidly.